On aircraft, undercarriages are generally extended in normal mode by means of a hydraulic circuit having hydraulic components adapted to work in sequence for unlocking and opening the undercarriage hatches, and then unlocking and extending the undercarriages, and possibly also for reclosing and relocking at least some of the hatches.
In the event of an undercarriage failing to extend, as a result of the failure of the hydraulic circuit or of an associated member, emergency means are provided for enabling the undercarriages to be extended.
This emergency mode is very often a mechanical mode. It consists in actuating the hatch latching boxes and the undercarriage latching boxes by means of rodding or cables actuated by the pilot using a release lever for releasing the hatches and the undercarriages so that they can then extend under the effect of gravity. Recently, proposals have been made to perform emergency actuation of latching boxes by using electric actuators. Proposals have also been made for a combined electrical and hydraulic mode that uses a pump driven by an electric motor to act for the purpose of unlatching the latching boxes by means of pressure.
Under all circumstances, emergency actuation constitutes a system in its own right that, in normal mode, is segregated and consequently requires a large amount of additional equipment. This results in an increase in weight and a reduction in the reliability of the system for extending undercarriages, and also to an associated high cost.